The present invention relates to a circuit arrangement for tuning into a television signal from among a number of radioelectric signals received by a television set which includes a picture display device. The arrangement is operable in at least two different modes which permit the selection of the television signal and includes, a controllable oscillator the frequency of which is determined by a control loop depending on a binary number N supplied by a processing unit on the basis of coded input data, the loop comprising frequency-dividing means for obtaining from the controllable oscillator a first signal dependent on the number N, means for comparing the said first signal with a second frequency reference signal for obtaining a third signal for required tuning, memory means with a number of cells for memorizing, in digital form, data relative to the signals to be tuned and, finally, means for supplying the number N both in direct response to the said coded input data and by calling up data from the memory means. The system commonly used on television receivers for tuning into the required channels is the so-called FREQUENCY SYNTHESIZER system. This system, made possible by the advent of integrated circuits, offers a number of advantages over other known systems, such as the conventional potentiometer type MECHANICAL MEMORY systems and the more recent so-called VOLTAGE SYNTHESIZER systems. The frequency synthesizer system is fully electronic enabling any channel to be called up directly by the user who formulates the channel number on a keyboard or other control device. The system usually comprises a quartz-controlled reference oscillator, a phase lock loop, a programmable divider and a computer which supplies the number to be sent to the programmable divider in response to the number of the channel set by the user.
Thanks to the phase lock loop, for each channel number set by the user, the frequency of the local oscillator on the set is kept so stable and accurate that the set is tuned to the corresponding channel signal with great precision. Further details about frequency synthesizer tuning systems can be found in the article entitled "A Frequency Synthesizer for Television Receivers" by E. G. Breeze, published in the November, 1974 issue of the "Transactions BTR" magazine, or "Digital Television Tuner Uses MOS LSI and Non Volatile Memory" by L. Penner, published in the Apr. 1, 1976 issue of "Electronics".
The frequency synthesizer system lends itself well to a number of different modes of television channel tuning:
(1) direct selection by formulating the required channel number as described above (television channels are numbered: for example, in the European C.C.I.R. standard, VHF band channels are numbered from 2 to 12 and UHF band channels from 21 to 69; in the American standard, VHF channels are numbered from 2 to 13 and UHF from 14 to 83). PA1 (2) memory selection: each of a certain set of keys corresponds to a preselected and memorized channel; PA1 (3) automatic scanning of all the channels of a given standard, or of all the channels contained in the memory or continuous scanning of all the frequency bands involved.
The first application enables immediate, direct selection of any one of the channels in the relative standard (60 in Europe, 82 in America).
The second enables faster detection of one of a limited number of preferred channels.
The third is a fast, simple way of finding out which standard channels can be received, which channels have been memorized and whether other broadcasting stations exist on non-standard frequencies such as the private broadcasting stations in Italy (there are currently over a hundred operating).
Examples of frequency synthesizer systems with this wide range of selection modes are described in West German Patent Application No. 26 45 833 and 26 52 185 and, in particular, Italian Patent Application No. 69.950-A/77 filed on Dec. 30, 1977 by the present applicant.
All these modes, which are particularly useful in areas where a number of broadcasting stations can be received, require highly complex control equipment which many users may find difficulty in operating. This is even more so if, in addition to emitter selection and standard receiver adjustment controls (volume, brightness, color, etc.), provision is made for additional accessory functions such as a digital clock which requires additional setting controls. What is more, it must be possible to preserve memorized channel data in the case of power outages. This can be done in the known way using a permanent external memory (battery-supplied low-consumption CMOS memory or non-volatile MNOS memory). Nevertheless, it has been shown that part of the data in the memory may be lost during the transient state between loss and return of power supply.
The aim of the present invention is to provide a circuit arrangement for a television receiver enabling the many functions described above to be affected simply and cheaply with a little operating difficulty as possible on the part of the user.
A further aim of the present invention is to provide a circuit arrangement designed to prevent loss of part of the data in the memory during the transient state between loss and return of power supply.